A rapid and widespread use of radio apparatus such as cellular phones has increased a demand for down-sized and light-weight resonators, and filters manufactured of a combination of such resonators. In these apparatus, dielectric filters and surface acoustic wave (SAW) filters have been mainly used. Recently, filters manufactured of piezoelectric thin film resonators draw attention, because the piezoelectric thin film resonator features a low loss in a high-frequency region, high power durability, excellent Electrostatic Discharge (ESD) characteristics, and permits a small size and a monolithic design.
Film Bulk Acoustic Resonator (FBAR) type resonator is known as one of such piezoelectric thin film resonators. This resonator includes as a main element a laminated structure composed of a lower electrode, a piezoelectric layer, and an upper electrode, and an opening (via hole or cavity) is located beneath a portion (membrane region) of the lower electrode where the lower electrode faces the upper electrode.
If an electrical signal of a high frequency is applied between the upper electrode and the lower electrode, an acoustic wave is excited within the piezoelectric film between the upper electrode and the lower electrode by the inverse piezoelectric effect. Conversely, the piezoelectric effect converts a distortion caused by the acoustic wave into an electrical signal. Since the acoustic wave is totally reflected from each surface of the upper and the lower electrodes in contact with air, the acoustic wave is transformed into a vibration in a thickness longitudinal mode in which a main displacement is in the direction of thickness of the piezoelectric layer. In such a structure, assuming that a thickness H is a total thickness of the thin film structure having as a main element the upper electrode, piezoelectric film, lower electrode formed on the opening, a resonance takes place at a frequency where the thickness H is equal to an integer multiple (n times) of a ½ wavelength of the acoustic wave. Let V represent a propagation velocity of the acoustic wave determined by a material, a resonance frequency F is written as F=nV/2H. Accordingly, a resonator having desired frequency characteristics is produced by controlling the resonance frequency by the thickness on the basis of the resonance phenomenon, and a filter is produced by connecting a plurality of resonators.
The electrode can be manufactured of one selected from or a combination of aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), titanium (Ti), etc. The combination is preferable to be used in a laminated form.
Silicon, glass, GaAs, or the like may be used for a substrate. In case of Si as the substrate, the opening may be formed by etching (wet etching or dry etching) a rear side of the Si substrate, or by wet etching a sacrificial layer formed on a front side of the Si substrate. In this specification, a hole penetrating from the rear side of the substrate through to the front side of the substrate is referred to as a “via hole” and the opening presents right beneath the lower electrode in the vicinity of the front side of the substrate is referred to as a “cavity or opening.” The known piezoelectric thin film resonators are divided into a via-hole type and a cavity type.
FIG. 20 is a sectional view of a known via-hole type piezoelectric thin film resonator. The structure illustrated in FIG. 20 is an example of a piezoelectric thin film resonator, and is disclosed in Electron. Lett., 17 (1981), pp. 507-509. In this structure, a lower electrode of Au—Cr 22, a piezoelectric film of ZnO 23, and an upper electrode of Al 24 are formed on a (100) Si substrate 21 having a thermally oxided layer (SiO2) 25. The via hole 26 is formed by performing an isotropic etching operation on the rear side of the Si substrate 21 using KOH aqueous solution or EDP aqueous solution (mixture of ethylenediamine, pyrocatechol, and water).
On the other hand, FIG. 21 is a sectional view of a cavity-type piezoelectric thin film resonator In the cavity-type piezoelectric thin film resonator, an upper electrode 34, piezoelectric film 33, and lower electrode 32 are formed as main elements on a sacrificial layer 35, and a cavity is finally formed by removing the sacrificial layer through etching. The piezoelectric thin film resonator illustrated in FIG. 21 is disclosed in Japanese Laid-open Patent Publication No. 6-40611. In this example, an island-like sacrificial pattern of ZnO is formed as a sacrificial layer, and a laminated structure composed of a dielectric film, upper electrode, piezoelectric film, lower electrode, and dielectric film is produced on the sacrificial pattern, and the sacrificial layer is then removed by acid to form the cavity (air bridge structure).
With reference to FIG. 22, Japanese Laid-open Patent Publication No. 2000-69594 discloses a piezoelectric thin film resonator in which a recess is formed on the front side of a substrate below a laminated region composed of an upper electrode, a piezoelectric film, and a lower electrode. In the piezoelectric thin film resonator illustrated in FIG. 22, a sacrificial layer 45 is deposited on a recess formed beforehand, the surface of the substrate 41 is then planarized. Then the laminated structure composed of the upper electrode 44, the piezoelectric film 43, the lower electrode 42 is formed on the planarized substrate 41, and finally the sacrificial layer is removed by etching in order to form the cavity.
Aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), and the like may be used for the piezoelectric film. In many cases, AlN is used from the standpoint of sound speed, temperature characteristics, quality value (Q value), and ease of film formation. In particular, the formation of the AlN film having crystallization highly oriented in c-axis (in a direction vertical to the surface of the lower electrode) is one of important factors in the determination of resonance characteristics and directly affects a coupling coefficient and Q value. On the other hand, the formation of the AlN film having crystallization highly oriented in c-axis needs a high-level energy to be applied during the film formation. For example, substrate heating needs to be performed at 1000° C. or higher in metal organic chemical vapor deposition (MOCVD), and substrate heating needs to be performed at 400° C. or higher in addition to power for plasma in plasma enhanced chemical vapor deposition (PECVD). With sputtering technique, substrate temperature rising due to sputtering to an insulating film is known. For this reason, the AlN film has generally a strong film stress (a remaining stress). As a result, a piezoelectric thin film resonator including the AlN film having crystallization highly oriented in c-axis suffers from problems such as an open-circuit at a leading part from the upper electrode and membrane destruction. In the piezoelectric thin film resonator, the cavity is preferably designed to include a membrane region so that resonance is properly performed, because an acoustic wave resonates in the membrane region.
Techniques to move a lower electrode from within a cavity to outside the cavity on a substrate have been introduced to overcome the problem of a degradation in mechanical strength (disclosed in Japanese Laid-open Patent Publication No. 2002-140075)
Although the structure disclosed in Japanese Laid-open Patent Publication No. 2002-140075 still suffers from the trade-off relationship that mechanical strength and Q value are improved while a drop in an electromechnical coupling coefficient is unavoidable.